1. Field of the Invention
The present invention relates to a receiver equipped with a plurality of antennas.
2. Description of the Related Art
Movable receivers, such as receivers mounted in vehicles, operate in an environment where the movement of the receiver causes a constant change in the reception conditions for radio waves. For this reason, such receivers use a technique called diversity reception where a single broadcast wave or a set of broadcast waves is received using a plurality of antennas and adjustments are made to try and maintain favorable reception conditions.
To facilitate mounting in a vehicle, vehicle-mounted receivers include antenna units and a main reception unit, with high-frequency cables being used to connect the two types of units and transfer reception waves between them. Each antenna unit is composed of an antenna for receiving broadcast waves and a low noise amplifier (hereafter abbreviated to “LNA”) that is positioned close to the antenna and amplifies the received broadcast waves. The main reception unit is composed of a demodulation/reproduction circuit that detects and demodulates the received broadcast waves and then reproduces the received information.
To perform diversity reception where a plurality of antennas are used, each antenna unit needs to be connected to the main reception unit with a separate high-frequency cable. This results in an increase in the number of high-frequency cables, which makes the receiver less practical. For this reason, conventional receivers are provided with a signal superposing circuit that gathers together the broadcast waves received by each antenna unit, with the signal superposing circuit and main reception unit being connected by a single high-frequency cable.
When the above construction is used, however, the broadcast waves received by all of the antenna units that can be used when performing diversity reception are superposed by the signal superposing circuit. As a result, noise that is produced in the LNA provided in each antenna unit and does not form part of the broadcast waves received by each antenna is also superposed by the signal superposing circuit. When the reception conditions are favorable, the reception signal has a sufficiently high level relative to this noise, so that there is little risk of the composite noise in the superimposed signal produced by the signal superposing circuit affecting the reproduction of the reception signal. However, when the reception conditions deteriorate, a high signal-to-noise ratio cannot be maintained for the reception signal, so that the composite noise can adversely affect the quality of the reception signal. In the worst-case scenario, the main reception unit can have difficulty in reproducing the reception signal.